MINI REVIEW ARTICLE published: 19 February 2015 doi: 10.3389/fpls.2015.00072 The green seaweed Ulva: a model system to study morphogenesis Thomas Wichard 1*, Bénédicte Charrier 2,3, Frédéric Mineur 4, John H. Bothwell 5, Olivier De Clerck 6 and Juliet C. Coates 7 1 Institute for Inorganic and Analytical Chemistry, Jena School for Microbial Communication, Friedrich Schiller University Jena, Jena, Germany 2 UMR 8227, Integrative Biology of Marine Models, Station Biologique de Roscoff, Centre National de la Recherche Scientifique, Roscoff, France 3 UMR 8227, Integrative Biology of Marine Models, Station Biologique de Roscoff, Sorbonne Universités, UPMC University of Paris 06, Roscoff, France 4 School of Biological Sciences, Queen’s University of Belfast, Belfast, UK 5 School of Biological and Biomedical Sciences and Durham Energy Institute, Durham University, Durham, UK 6 Phycology Research Group and Center for Molecular Phylogenetics and Evolution, Ghent University, Ghent, Belgium 7 School of Biosciences, University of Birmingham, Birmingham, UK Edited by: Green macroalgae, mostly represented by the Ulvophyceae, the main multicellular branch Kimberley C. Snowden, of the Chlorophyceae, constitute important primary producers of marine and brackish The New Zealand Institute for Plant and Food Research Limited, coastal ecosystems. Ulva or sea lettuce species are some of the most abundant New Zealand representatives, being ubiquitous in coastal benthic communities around the world. Reviewed by: Nonetheless the genus also remains largely understudied. This review highlights Ulva Burkhard Becker, University of as an exciting novel model organism for studies of algal growth, development and Cologne, Germany morphogenesis as well as mutualistic interactions. The key reasons that Ulva is potentially Yin-Long Qiu, University of Michigan, USA such a good model system are: (i) patterns of Ulva development can drive ecologically *Correspondence: important events, such as the increasing number of green tides observed worldwide as Thomas Wichard, Institute for a result of eutrophication of coastal waters, (ii) Ulva growth is symbiotic, with proper Inorganic and Analytical Chemistry, development requiring close association with bacterial epiphytes, (iii) Ulva is extremely Jena School for Microbial developmentally plastic, which can shed light on the transition from simple to complex Communication, Friedrich Schiller University Jena, Lessingstr. 8, multicellularity and (iv) Ulva will provide additional information about the evolution of the 07743 Jena, Germany green lineage. e-mail: [email protected] Keywords: algal genetics, chlorophyta, green tides, holobiont, multicellular organism, model organism INTRODUCTION mechanisms underlying morphological diversity, which is also The marine seaweed Ulva belongs to the chlorophytes, an infor- influenced by associated bacteria via cross-kingdom cross-talk. mal assemblage of three traditional classes (Ulvo-, Trebouxio- and Within the order Ulvales, where all species have uninucleate cells, Chlorophyceae) that evolved from unicellular marine planktonic algae present simple morphologies (Brodie et al., 2007). The prasinophyte algae in the Neoproterozoic (Herron et al., 2009; Ulvaceae stand out by their organization into a basic “diptych” Verbruggen et al., 2009; Parfrey et al., 2011). Although recently plan, either tubular thalli (e.g., Blidingia) or flattened distromatic considerable doubts have arisen regarding the monophyly of the (2 cells thick) blades (e.g., Umbraulva). Both morphological three classes making up the core Chlorophytes (Zuccarello et al., forms are present in the genus Ulva [“enteromorpha” (tubular) 2009; Fucikova et al., 2014; Lemieux et al., 2014), for the sake of and “sea lettuces” (flattened)], appearing concomitantly in clarity we will refer to them by their traditional names, unless oth- many sub-clades reported in molecular phylogenies of the genus erwise specified. The Chloro- and Trebouxiophyceae diversified (Hayden and Waaland, 2002; Hayden et al., 2003), including at largely in freshwater and terrestrial habitats, while Ulvophyceae the species level (e.g., Ulva mutabilis and U. compressa; Løvlie, came to dominate shallow marine environments (Becker and 1964; Tan et al., 1999; see Figure 1A). Marin, 2009). Ulvophyceae display an astounding morphological From an economic perspective, green seaweeds are sus- and cytological diversity (Cocquyt et al., 2010). This includes tainable biomass feedstocks for the food and biotech indus- unicells, filaments, sheet-like thalli (vegetative shoot-like tissues) tries, including bioremediation, integrated aquaculture systems and giant-celled coenocytic or siphonal seaweeds (Mine et al., and potential biofuel production (Nisizawa et al., 1987; Neori 2008; Cocquyt et al., 2010), which branch and fuse to form mor- et al., 1996, 2004; Dibenedetto, 2012; Alsufyani et al., 2014). phologies with root-, stem- and leaf-like structures comparable in Ulva is increasingly important in coastal ecosystem man- size to large shrubs on land (Chisholm et al., 1996; Vroom and agement, due to eutrophication-driven green tides in shal- Smith, 2003; Littler et al., 2005). low environments (Figure 1B; Leliaert et al., 2009; Teichberg The Ulvophyceae thus form an excellent group of organisms et al., 2010; Gosch et al., 2012; Smetacek and Zingone, in which to elucidate the evolutionary processes and genetic 2013). www.frontiersin.org February 2015 | Volume 6 | Article 72 | 1 Wichard et al. Model organism Ulva the gametogenesis and subsequent gamete release, independently of photoperiod (Nilsen and Nordby, 1975; Stratmann et al., 1996; Wichard and Oertel, 2010). Vegetative thalli release a high molecular mass cell wall glycoprotein (SI-1) into the surround- ing medium while containing a second low molecular weight inhibitor (SI-2) in the space between the two cell layers of the thallus. The transformation from a blade cell into a gametangium occurs only if SI-1 levels drop and the constantly-present SI-2 is no longer perceived by the alga, as discussed by Stratmann et al. (1996). Facilitating the potential of Ulva as a model organism, game- togenesis can be induced artificially by removal of both SI, via cutting the thallus into single-layer fragments and subsequently washing, as exemplified originally for U. mutabilis by Stratmann et al.(1996), but also observed in U. lactuca, U. linza, and U. rigida (Stratmann et al., 1996; Wichard and Oertel, 2010; Vesty et al., 2015). After induction, gametes are released by removal of the SWI (accumulated during gametogenesis) synchronizing the discharge of the gametangia and increasing the mating probability (Wichard and Oertel, 2010). Moreover, unmated gametes can develop parthenogenetically into clonal, haploid gametophytes ideal for genetic manipulation and reproducible standardized experiments. The generation time of U. mutabilis is short: only 3– FIGURE 1 | (A) The worldwide distribution of U. compressa and related populations including U. mutabilis (black circles; rbcL haplotypes available 5 weeks’ growth is required between potential inducibility of syn- through NCBI GenBank) are presented as an example of the cosmopolitan chronous gametogenesis (Løvlie, 1964; Stratmann et al., 1996). nature of Ulva spp. The sea surface temperature map was plotted using The sporulation-inhibitor-regulated life-cycle transition may Bio-ORACLE1 (Tyberghein et al., 2012). (B) Ulva can cause green tides, e.g., have strong relevance to the dynamics of green tide formations, as in the lagoon Ria Formosa (Portugal). Photo is a courtesy of Dr. Eric-Jan Malta (IFAPA, Spain). fragmentation is often pivotal during algal bloom succession (Gao et al., 2010). The aim of this review is to summarize key features of Ulva, to SYMBIOTIC NATURE OF Ulva GROWTH stress understudied fundamental questions in algal developmen- Cross-kingdom cross-talk between macroalgae and bacteria con- tal biology, and to highlight new perspectives on the “Ulva genetic trols algal settlement, growth and development (Joint et al., 2002, tool kit.” 2007). Several studies have shown that Ulva fails to form its typical morphology in the absence of the appropriate bacteria and simply REGULATION AND MANIPULATION OF THE LIFE CYCLE proliferates as an undifferentiated clump of callus cells (e.g., Fries, Although summarized as a simple alternation of isomorphic 1975; Marshall et al., 2006; Spoerner et al., 2012). generations, the “haplodiplontic” life-cycle of many Ulva species Interactions between Ulva spp. and their associated bacteria is generally more complex (Føyn, 1958; Hoxmark, 1975; Phillips, have been well-characterized over the last 50 years and the bac- 1990) and has been extensively investigated in U. lactuca and terial colonization of Ulva species has been defined based on U. mutabilis. The two macroscopic stages, the sporophyte and 16S rDNA gene phylogeny (Burke et al., 2009; Lachnit et al., gametophyte, can each originate in more than one way. Diploid 2009). Burke et al.(2011a) showed that the algal microbiota of multicellular sporophytes can originate from the fusion of two U. australis varies over the season, and between very close sample gametes of opposite mating type. Haploid gametophytes can sites. Although they did not rigorously verify the mono-specificity derive from meiotically-formed haploid zoids or from unmated of their Ulva samples, they concluded that Ulva does not possess a biflagellated gametes. In addition, diploid parthenosporophytes core microbial